In Reply.—
Dr Taylor's "time machine" provides a valuable opportunity to clarify the importance of reliability and validity as they pertain to concepts such as schizophrenia and to diagnostic or classification systems in general. Dr Taylor, whose own work has been influential in contributing to the trend toward diagnoses that are more "reliable"1 is justifiably proud of the newer diagnostic systems.Reliability, which has been emphasized in the construction of these newer diagnostic systems, such as DSM-III, refers to the consistency of a test or measure. For diagnosis, reliability refers to the consistency of a diagnosis across or within clinical settings.For a system of classifying schizophrenia, one would assess whether the same types of patients are being classified as "schizophrenic" in different settings.Reliability is one of several important features for a diagnostic system. Validity, which is even more important, involves the degree to which a test or measure